Methods of Manufacturing Bentonite Polution Control Sorbents

ABSTRACT

Methods of manufacturing bentonite sorbents for removal of pollutants including mercury from gas streams, such as a flue gas stream from coal-fired utility plants are disclosed. The methods include mixing bentonite sorbent particles with a sulfide salt and a metal salt to form a metal sulfide on the outer surface of the bentonite sorbent particles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/291,091, filed on Nov. 30, 2005, the content of which is herebyincorporated by reference in its entirety.

TECHNICAL FIELD

Embodiments of the invention relate methods for the manufacture ofbentonite pollution control sorbents.

BACKGROUND ART

Emission of pollutants, for example, mercury, from sources such ascoal-fired and oil-fired boilers has become a major environmentalconcern. Mercury (Hg) is a potent neurotoxin that can affect humanhealth at very low concentrations. The largest source of mercuryemission in the United States is coal-fired electric power plants.Coal-fired power plants account for between one-third and one-half oftotal mercury emissions in the United States. Mercury is foundpredominantly in the vapor-phase in coal-fired boiler flue gas. Mercurycan also be bound to fly ash in the flue gas.

On Dec. 15, 2003, the Environmental Protection Agency (EPA) proposedstandards for emissions of mercury from coal-fired electric powerplants, under the authority of Sections 111 and 112 of the Clean AirAct. In their first phase, the standards could require a 29% reductionin emissions by 2008 or 2010, depending on the regulatory option chosenby the government. In addition to EPA's regulatory effort, in the UnitedStates Congress, numerous bills recently have been introduced toregulate these emissions. These regulatory and legislative initiativesto reduce mercury emissions indicate a need for improvements in mercuryemission technology.

There are three basic forms of Hg in the flue gas from a coal-firedelectric utility boiler: elemental Hg (referred to herein by the symbolHg⁰); compounds of oxidized Hg (referred to herein by the symbol Hg²⁺);and particle-bound mercury. Oxidized mercury compounds in the flue gasfrom a coal-fired electric utility boiler may include mercury chloride(HgCl₂), mercury oxide (HgO), and mercury sulfate (HgSO₄). Oxidizedmercury compounds are sometimes referred to collectively as ionicmercury. This is because, while oxidized mercury compounds may not existas mercuric ions in the boiler flue gas, these compounds are measured asionic mercury by the speciation test method used to measure oxidized Hg.The term speciation is used to denote the relative amounts of thesethree forms of Hg in the flue gas of the boiler. High temperaturesgenerated by combustion in a coal boiler furnace vaporize Hg in thecoal. The resulting gaseous Hg⁰ exiting the furnace combustion zone canundergo subsequent oxidation in the flue gas by several mechanisms. Thepredominant oxidized Hg species in boiler flue gases is believed to beHgCl₂. Other possible oxidized species may include HgO, HgSO₄, andmercuric nitrate monohydrate (Hg(NO₃)₂.H₂O).

Gaseous Hg (both Hg⁰ and Hg²⁺) can be adsorbed by the solid particles inboiler flue gas. Adsorption refers to the phenomenon where a vapormolecule in a gas stream contacts the surface of a solid particle and isheld there by attractive forces between the vapor molecule and thesolid. Solid particles are present in all coal-fired electric utilityboiler flue gas as a result of the ash that is generated duringcombustion of the coal. Ash that exits the furnace with the flue gas iscalled fly ash. Other types of solid particles, called sorbents, may beintroduced into the flue gas stream (e.g., lime, powdered activatedcarbon) for pollutant emission control. Both types of particles mayadsorb gaseous Hg in the boiler flue gas.

Sorbents used to capture mercury and other pollutants in flue gas arecharacterized by their physical and chemical properties. The most commonphysical characterization is surface area. The interior of certainsorbent particles are highly porous. The surface area of sorbents may bedetermined using the Brunauer, Emmett, and Teller (BET) method of N₂adsorption. Surface areas of currently used sorbents range from 5 m²/gfor Ca-based sorbents to over 2000 m²/g for highly porous activatedcarbons. EPA Report, Control of Mercury Emissions From Coal-FiredElectric Utility Boilers, Interim Report, EPA-600/R-01-109, April 2002.For most sorbents, mercury capture often increases with increasingsurface area of the sorbent.

Mercury and other pollutants can be captured and removed from a flue gasstream by injection of a sorbent into the exhaust stream with subsequentcollection in a particulate matter control device such as anelectrostatic precipitator or a fabric filter. Adsorptive capture of Hgfrom flue gas is a complex process that involves many variables. Thesevariables include the temperature and composition of the flue gas, theconcentration of Hg in the exhaust stream, and the physical and chemicalcharacteristics of the sorbent. Of the known Hg sorbents, activatedcarbon and calcium-based sorbents have been the most actively studied.

Currently, the most commonly used method for mercury emission reductionis the injection of powdered activated carbon into the flue stream ofcoal-fired and oil-fired plants. Currently, there is no availablecontrol method that efficiently collects all mercury species present inthe flue gas stream. Coal-fired combustion flue gas streams are ofparticular concern because their composition includes trace amounts ofacid gases, including SO₂ and SO₃, NO and NO₂, and HCl. These acid gaseshave been shown to degrade the performance of activated carbon. Thoughpowdered activated carbon is effective to capture oxidized mercuryspecies such as Hg⁺², powdered activated carbon (PAC) is not aseffective for elemental mercury which constitutes a major Hg species influe gas, especially for subbituminous coals and lignite. There havebeen efforts to enhance the Hg⁰ trapping efficiency of PAC byincorporating bromine species. This, however, not only introducessignificantly higher cost, but a disadvantage to this approach is thatbromine itself is a potential environmental hazard. Furthermore, thepresence of PAC hinders the use of the fly ash for cement industry andother applications due to its color and other properties.

As noted above, alternatives to PAC sorbents have been utilized toreduce mercury emissions from coal-fired boilers. Examples of sorbentsthat have been used for mercury removal include those disclosed inUnited States Patent Application Publication No. 2003/0103882 and inU.S. Pat. No. 6,719,828. In United States Patent Application PublicationNo. 2003/0103882, calcium carbonate and kaolin from paper mill wastesludge were calcined and used for Hg removal at high temperatures above170° C., preferably 500° C. U.S. Pat. No. 6,719,828 teaches thepreparation of layered sorbents such as clays with metal sulfide betweenthe clay layers and methods for their preparation. The method used toprepare the layered sorbents is based on an ion exchange process, whichlimits the selection of substrates to only those having high ionexchange capacity. In addition, ion exchange is time-consuming andinvolves several wet process steps, which significantly impairs thereproducibility, performance, scalability, equipment requirements, andcost of the sorbent. For example, a sorbent made in accordance with theteachings of U.S. Pat. No. 6,719,828 involves swelling a clay in anacidified solution, introducing a metal salt solution to exchange metalions between the layers of the clay, filtering the ion exchanged clay,re-dispersing the clay in solution, sulfidation of the clay by addinganother sulfide solution, and finally the product is filtered and dried.Another shortcoming of the process disclosed in U.S. Pat. No. 6,719,828is that the by-products of the ion exchange process, i.e., the wastesolutions of metal ions and hydrogen sulfide generated from the acidicsolution, are an environmental liability.

There is an ongoing need to provide improved pollution control sorbentsand methods of their manufacture. It would be desirable to providesorbents containing metal sulfides on the sorbent substrate that can bemanufactured easily and inexpensively. In this regard, simple andenvironmentally friendly methods that effectively disperse metal sulfideon readily available substrates, which do not require the numerous stepsinvolved in an ion exchange process are needed.

SUMMARY

Aspects of the invention include methods of manufacturing bentonitesorbents for removal of pollutants such as heavy metals from gasstreams. The sorbents are useful for, but not limited to, the removal ofmercury from flue gas streams generated by the combustion of coal.

In a first aspect, a method of making sorbent particles for the removalof mercury from a gaseous stream is provided. The method comprisesmixing a metal salt with bentonite particles by grinding or milling;mixing a sulfide salt with the bentonite particles and the metal salt;and drying the mixture. In certain embodiments, the method may furtherinclude reducing the particle size of the particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is graph showing mercury removal versus copper loading on acopper weight percent basis; and

FIG. 2 is a graph showing the effect of the addition of chloride saltson mercury removal.

DETAILED DESCRIPTION

Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced orbeing carried out in various ways.

One aspect of the present invention relates to methods of manufacturingbentonite sorbents. Bentonite is an aluminum phyllosilicate clayconsisting mostly of montmorillonite,(Na,Ca)_(0.33)(Al,Mg)₂Si₄O₁₀(OH)₂.nH₂O, which may also be referred to asFuller's earth or montmorillonite. Applicants have determined that anion exchange process such as the type disclosed in U.S. Pat. No.6,719,828, is not necessary for mercury capture by the bentonite sorbentmaterial.

According to one or more embodiments, incipient wetness or solid-statereactive grinding processes are used to disperse metal sulfide salts onthe surface of bentonite sorbent particles. The incipient wetness orsolid-state reactive grinding methods disclosed herein do not requireexcessive liquids associated with wet processes, thus eliminatingproblems associated with wet processes. These problems may includedisposal of waste of metal ions solution or hydrogen sulfide. Inaddition, certain embodiments of the present invention provide accuratecontrol of the amount of metal sulfide on the surface of the sorbent.Further, the processes according to certain embodiments are much fasterand significantly reduce the equipment and resources required for thelarge-scale production. Moreover, the highly dispersed metal sulfide onthe surface of the sorbent provides better contact between Hg and metalsulfide when used for mercury removal, as evidenced by the higher andfaster mercury capture than those obtained by the ion exchange processwhen measured by an in-flight test with simulated flue gases.

Another embodiment of the invention pertains to the addition of at leastone halogen-containing salt to the sorbent. The presence of chloridesignificantly promotes the Hg-capture, possibly due to the formation ofmercury chloride complexes.

According to one embodiment of the invention, the steps for makingsurface-dispersed metal sulfide include: mixing a metal salt with abentonite substrate particle by grinding or milling; adding and mixing,preferably by grinding, a sulfide salt with the metal salt and substrateparticle mixture; and drying the mixture. In certain embodiments, theresultant material is optionally milled to the desired particle size.For low metal sulfide loading, the sulfide can be added by incipientwetness as described below. For high metal sulfide loading, the sulfidecan be added by solid-state grinding.

While embodiments of the present invention should not be limited by aparticular theory of operation or scientific principle, it is believedthat the metal salt (e.g., CuCl₂) reacts with sulfide (e.g., Na₂S) insitu to form a metal sulfide (for example, CuS) on the surface of thesubstrate, particles (for example, kaolin clay). The formation of themetal sulfide can occur either by contact via incipient wetness of thereaction or said solid-state reactive grinding. Since most metal saltsand sulfides are crystal hydrates, for high metal sulfide loading, thewater released from the chemicals during mixing is sufficient to moistenthe mixture. As an example, the following reaction,

CuCl₂.2H₂O+Na₂S.9H₂O→CuS+2NaCl+11H₂O,

demonstrates that no additional water is required to disperse CuS in themixture.

The metal salts used according to method embodiments include any metalsalt that can release a metal ion with any oxidation states when thesalt contacts a sulfide salt and thereafter forms water insoluble metalsulfide on the surface of a substrate. The metal includes alkaline earthmetals and the metals that have an atomic number of between 21 and 30,between 38 and 50, and between 56 and 79 and combinations thereof.Examples of metals include copper, titanium, tin, iron, and manganese. Apresently preferred metal ion is Cu⁺² and presently preferred salts arenitrate, chloride, sulfate, and acetate and combinations thereof. Theloading level of metal is between about 0 and 100 weight percent,preferably between about 1 and 50 weight percent, and most preferablybetween about 1 and 20 weight percent.

Any sulfide precursor that forms the S⁻² anion can be used in accordancewith embodiments of the invention. This includes, but is not limited to,Na₂S and (NH₄)₂S. In a specific embodiment, the sulfide precursor isNa₂S. Sulfide loading level can be stoichiometric (1:1 atomic ratio) ordifferent than that of the metal ion.

Dispersion of the metal sulfide can be accomplished by any method aslong as the metal sulfide is well dispersed on the surface of thesubstrate. Such methods include, but are not limited to, incipientwetness, solid-state mixing, spray-drying, sprinkling of solution on thesolid, precipitation, co-precipitation, etc. Detailed embodiments usesolid-state reactive grinding for high metal sulfide loading, incipientwetness for low metal sulfide loading, or a combination of grinding andincipient wetness. The order of adding the metal salts and sulfide saltscan be altered, e.g., the sulfide salt can be added to the substratefirst followed by addition of the metal salt. The metal sulfide can beadded to the substrate one salt at a time (e.g., add CuCl₂ first,followed by adding Na₂S), two salts at the same time (e.g.,co-precipitation), or directly mixing a fine metal sulfide powder withthe substrate.

Additional steps according to embodiments of the invention may includedrying and milling the sorbent. Drying may be accomplished by any meanssuch as static, spray-drying, microwave drying, or on a moving belt at atemperature in the range of about 25° and 200° C. for 0 to 15 hours. Indetailed embodiments, drying occurs within the temperature range ofabout 60° and 150° C. In further detailed embodiments, drying occurs inthe range of about 90° and 140° C. The sorbent can optionally be milledto an average particle size below about 80 μm, preferably below about 40μm.

Without intending to limit the invention in any manner, the presentinvention will be more fully described by the following examples.

EXAMPLES

Several samples were prepared in accordance with the methods formanufacturing sorbent substrates described above. Table 1 lists thesample number, the sulfide salt formed on the surface of the sorbent(D), the sorbent substrate (A), the metal salt (B), and the precursorsulfide salt (C). The last column of Table 1 indicates the order ofmixing of each of the ingredients. For example, A-B-C indicates that themetal salt (B) was added to the substrate (A) first, and then precursorsulfide salt (C) was added to the mixture.

For example, sample ECS22, 1.18 g of CuCl₂.2H₂O was mixed with 10.0 g ofbentonite by a thorough solid-state grinding. Then 1.67 g of Na₂S.9H₂Owas dissolved in de-ionized water and added to the solid mixture by whatis termed herein as an incipient wetness process, in which the solutionwas added drop-wise to the solid mixture which was stirred rigorously.The resultant moistened solid was wet enough to completely disperse CuSon the bentonite, but dry enough so that the paste did not flow. Themoistened paste was then dried at 105° C. in air overnight and milled toa particle size of D₉₀<10 μm.

For sample ECS24, 2.44 g of CuCl₂.2H₂O was mixed with 10.0 g ofbentonite by a thorough solid-state grinding Then, 2.91 g of Na₂S.9H₂Opowder was added in by another thorough solid-state grinding. Themoistened paste was then dried at 105° C. in air overnight and milled toa particle size of D₉₀<10 μm.

The remaining samples were prepared in a very similar way as the abovetwo samples. The source and purity of the raw chemicals are listed inTable 2, and the main characteristic properties of the sorbentsubstrates are listed in Table 3.

TABLE 1 Summary of the Hg-removal sorbent composition and preparationmethods Example Metal Sulfide (D) Substrate (A) Metal Salt (B) PrecursorSulfide (C) Prep ECS01 CuS (10% Cu basis) 10 g Na-bentonite 2.95 gCuCl₂•2H₂O 4.16 g Na₂S•9H₂O A-B-C solid-state grinding solid-stategrinding ECS02 CuS (10% Cu basis) 10 g Na-bentonite 4.32 g CuSO₄•5H₂O4.16 g Na₂S•9H₂O A-B-C solid-state grinding solid-state grinding ECS03CuS (20% Cu basis) 10 g Na-bentonite 5.9 g CuCl₂•2H₂O 8.33 g Na₂S•9H₂OA-B-C solid-state grinding solid-state grinding ECS04 Fe₂S₃ (6% Febasis) 10 g Na-bentonite 3.12 g FeCl₃•6H₂O 16 g Na₂S•9H₂O A-B-Csolid-state grinding solid-state grinding ECS06 CuS (20% Cu basis) + 10g Na-bentonite 5.9 g CuCl₂•2H₂O 2.70 g Na₂S•9H₂O A-B-C CuCl_(2,)solid-state grinding solid-state grinding ECS07 CuS (10% Cu basis) + 10g Na bentonite 2.95 g CuCl₂•2H₂O 0.56 g sulfur powder A-B-C sulfurpowder solid state grinding heated to 150° C. for 0.5 hr ECS08 MnS (4%Mn basis) 10 g Na-bentonite 1.71 g MnCl₂•4H₂O 4.16 g Na₂S•9H₂O A-B-Csolid-state grinding solid-state grinding ECS09 S (10% S basis) 10 gNa-bentonite 0 1.11 g sulfur powder A-B ECS10 none 10 g Na-bentonite 0 0A ECS11 MnS₂ (3% Mn basis) 10 g Na-bentonite 1.37 g KMnO₄ 8.33 gNa₂S•9H₂O A-B-C solid-state grinding solid-state grinding ECS12 KMnO₄(4% Mn basis) 10 g Na-bentonite 1.37 g KMnO₄ solid-state grinding A-Bsolid-state grinding ECS13 AgNO₃ (1% Ag basis) 10 g Na-bentonite 0.159 gAgNO₃ 0 A-B ECS15 CuS (20% Cu basis) 10 g Na-bentonite 1.48 g CuCl₂•2H₂O2.08 g Na₂S•9H₂O A-B-C solid-state grinding solid-state grinding ECS20CuS (15% Cu basis) 10 g bentonite 4.43 g CuCl₂•2H₂O 6.25 g Na₂S•9H₂OA-B-C solid-state grinding solid-state grinding ECS21 CuS (10% Cu basis)20 g bentonite 0.90 g CuCl₂•2H₂O 8.33 g Na₂S•9H₂O A-B-C solid-stategrinding solid-state grinding (food processor blending) ECS22 CuS (4% Cubasis) 10 g bentonite 1.18 g CuCl₂•2H₂O 1.67 g Na₂S•9H₂O A-B-Csolid-state grinding incipient wetness ECS23 CuS (4% Cu basis) 10 gbentonite 1.72 g CuSO₄•5H₂O 1.67 g Na₂S•9H₂O A-B-C solid-state grindingincipient wetness ECS24 CuS (7% Cu basis) 10 g bentonite 2.44 gCuCl₂•2H₂O 2.91 g Na₂S•9H₂O A-B-C solid-state grinding solid-stategrinding ECS25 CuS (7% Cu basis) 10 g bentonite 3.01 g CuSO₄•5H₂O 2.91 gNa₂S•9H₂O A-B-C solid-state grinding solid-state grinding ECS26 CuS (10%Cu basis) 20 g bentonite 5.9 g CuCl₂•2H₂O 8.33 g Na₂S•9H₂O A-B-Csolid-state grinding blending by blender ECS27 CuS (10% Cu basis) + 10 gbentonite 4.32 g CuSO₄•5H₂O + 4.161 g Na₂S•9H₂O A-B-C MgCl₂ 1.00 g MgCl₂solid-state grinding solid-state grinding ECS28 CuS (10% Cu basis) + 10g bentonite 4.32 g CuSO₄•5H₂O + 4.16 g Na₂S•9H₂O A-B-C MgCl₂ 2.00 gMgCl₂ solid-state grinding solid-state grinding ECS29 CuS (10% Cubasis) + 10 g bentonite + 4.32 g CuSO₄•5H₂O + 4.16 g Na₂S•9H₂O A-B-CMgCl₂ 5.00 g MgCl₂ solid-state grinding solid-state grinding ECS30 CuS(10% Cu basis) 10 g bentonite 2.16 g CuSO₄•5H₂O + 4.16 g Na₂S•9H₂O A-B-C1.48 g CCl₂•2H₂O solid-state grinding solid-state grinding ECS31 CuS(10% Cu basis) 10 g bentonite 3.24 g CuSO₄•5H₂O + 0.74 g Na₂S•9H₂O A-B-C1.48 g CCl₂•2H₂O solid-state grinding solid-state grinding ECS32 CuS(10% Cu basis) 10 g bentonite 3.45 g Cu(acetate)₂•H₂O 16 g Na₂S•9H₂OA-B-C solid-state grinding solid-state grinding ECS33 CuS (7% Cu basis)10 g bentonite 2.42 g Cu(acetate)₂•H₂O 2.91 g Na₂S•9H₂O A-B-Csolid-state grinding solid-state grinding ECS34 CuS (4% Cu basis) 10 gbentonite 1.38 g Cu(acetate)₂•H₂O 1.66 g Na₂S•9H₂O A-B-C solid-stategrinding incipient wetness ECS35 CuS (100% Cu basis) 10 g bentonite 4.02g Cu(NO₃)₂• 5/2H₂O 16 g Na₂S•9H₂O A-B-C solid-state grinding solid-stategrinding ECS36 CuS (7% Cu basis) 10 g bentonite 2.82 g Cu(NO₃)₂• 5/2H₂O2.91 g Na₂S•9H₂O A-B-C solid-state grinding solid-state grinding ECS37CuS (4% Cu basis) 10 g bentonite 1.61 g Cu(NO₃)₂• 5/2H₂O 1.66 gNa₂S•9H₂O A-B-C solid-state grinding incipient wetness ECS38 CuS (10% Cubasis) 5 g bentonite 0.83 g CuS — A-B solid-state mixing ECS39 CuS (20%Cu basis) 5 g bentonite 1.66 g CuS — A-B solid-state mixing ECS40 CuS(1% Cu basis) 10 g Bentonite 0.295 g CuCl₂•2H₂O 0.416 g Na₂S•9H₂O A-C-Bincipient wetness solid-state grinding ECS41 CuS (2% Cu basis) 10 gBentonite 0.59 CuCl₂•2H₂O 0.832 g Na₂S•9H₂O A-C-B incipient wetnesssolid-state grinding ECS42 CuS (3% Cu basis) 10 g Bentonite 0.885 gCuCl₂•2H₂O 1.248 g Na₂S•9H₂O A-C-B incipient wetness solid-stategrinding ECS43 CuS (1% Cu basis) 10 g Bentonite 0.433 g CuSO₄•5H₂O 0.416g Na₂S•9H₂O A-C-B incipient wetness solid-state grinding ECS44 CuS (2%Cu basis) 10 g Bentonite 0.866 g CuSO₄•5H₂O 0.833 g Na₂S•9H₂O A-C-Bincipient wetness solid-state grinding ECS45 CuS (3% Cu basis) 10 gBentonite 1.299 g CuSO₄•5H₂O 1.248 g Na₂S•9H₂O A-C-B incipient wetnesssolid-state grinding ECS46 CuS (4% Cu basis) + 10 g Bentonite 1.732 gCuSO₄•5H₂O 1.664 g Na₂S•9H₂O A-B-C MgCl₂ 0.100 g MgCl₂ incipient wetnesssolid-state grinding ECS47 CuS (3% Cu basis) + 10 g Bentonite 1.299 gCuSO₄•5H₂O 1.248 g Na₂S•9H₂O A-B-C MgCl₂ 0.100 g MgCl₂ incipient wetnesssolid-state grinding ECS48 CuS (3% Cu basis) + 10 g Bentonite 1.299 gCuSO₄•5H₂O 1.248 g Na₂S•9H₂O A-B-C MgCl₂ 0.200 g MgCl₂ incipient wetnesssolid-state grinding ECS49 CuS (3% Cu basis) + 10 g Bentonite 1.299 gCuSO₄•5H₂O 1.248 g Na₂S•9H₂O A-B-C MgCl₂ 0.500 g MgCl₂ incipient wetnesssolid-state grinding ECS50 CuS (3% Cu basis) + 10 g Bentonite 1.299 gCuSO₄•5H₂O 1.248 g Na₂S•9H₂O A-B-C NaCl 0.100 g NaCl₂ incipient wetnesssolid-state grinding ECS51 CuS (3% Cu basis) + 10 g Bentonite 1.299 gCuSO₄•5H₂O 1.248 g Na₂S•9H₂O A-B-C NaCl 0.200 g NaCl₂ incipient wetnesssolid-state grinding ECS52 CuS (3% Cu basis) + 10 g Bentonite 1.299 gCuSO₄•5H₂O 1.248 g Na₂S•9H₂O A-B-C NaCl 0.500 g NaCl₂ incipient wetnesssolid-state grinding ECS57 Fe₂S₃ (10.0% S basis) 10 g Na-bentonite 6.24g FeCl₃•6H₂O 8.33 g Na₂S•9H₂O A-B-C solid-state grinding solid-stategrinding ECS58 CuS (5.0% S basis) 10 g bentonite 4.33 g CuSO₄•5H₂O 4.16g Na₂S•9H₂O A-B-C co-precipitation co-precipitation

TABLE 2 Source and purity of the raw chemicals Raw Chemical Supplier orSource Purity CuCl₂•2H₂O Alfa-Aesar 99+% CuSO₄•5H₂O Alfa-Aesar98.0-102.0% Cu(Acetate)₂•H₂O Aldrich 98+% Cu(NO₃)₂•5H₂O Aldrich  98%Na₂S•9H₂O Alfa-Aesar 98.0-103.0% FeCl₃•6H₂O Aldrich 98+% MnCl₂•4H₂OAldrich 98+% MgCl₂ Alfa-Aesar  99% KMnO₄ Alfa-Aesar 99.0% 

TABLE 3 Source Treatment and Propeties of Bentonite Substrate Supplieror Source Black Hill Bento Pre-treatment Milled to a particle size ofD₉₀ <10 μm N₂ Surface Area 35.4 m²/g N₂ Pore Volume 0.11 cc/g XRDCrystallinity Crystalline Structure Layered - Interlayer accessible PoreDiameter 12.2 nm Main Chemical Composition (%) SiO₂ Al₂O₃ Fe₂O₃ K₂O MgONa₂O TiO₂ CaO 67.50 20.20 3.20 0.51 2.06 2.64 0.23 2.42

The formation of metal sulfide (CuS) on the substrate particles isevidenced by the darkening of the paste and the heat release due to thefollowing process:

CuCl₂.2H₂O+Na₂S.9H₂O→CuS+2NaCl+11H₂O

Spectroscopic data also supports this observation of CuS formation onthe substrate. Infrared (IR) spectra of bentonite-supported chemicalCuCl₂, Na₂S, CuS, NaCl, and three sorbents that were prepared from thesolid-state grinding of CuCl₂.2H₂O, Na₂S.9H₂O and bentonite, asdescribed above, confirmed that the three sorbents (2.5, 5.0 and 10.0% Sbasis) contain predominantly CuS and NaCl and very little CuCl₂ and Na₂Sprecursors. X-ray powder diffraction provided similar evidence of theformation of CuS. X-ray elemental microprobe also showed that the spacedistribution of Cu on the surface of substrate particles is identical tothat of sulfur, indicating again the formation of CuS on the substratesurface.

Mercury Removal Evaluation

The Hg-removal performance of the sorbents described above was evaluatedby an in-flight test, which is a commonly used fast screening methodused to rank sorbents. The measurement includes the total mercuryremoval from simulated flue gas(Hg]_(injection start)−[Hg]_(injection stop))/[Hg]_(injection start)*100%)and the kinetics, or rate, of the Hg-capture (−d(Hg %)/dt). Bothparameters are important since the former measures the total Hg-capturewhile the latter is directly related to the strength of adsorption siteson the sorbent material. Thus, good sorption may be characterized byboth high and fast Hg removal at fixed sorbent injection rate.

Mercury concentration at the outlet of the sorbent injection chamber wasmeasured using an Ohio Lumex cold vapor atomic absorption instrument.The simulated flue gas consisted of 1600 ppm SO₂, 400 ppm NOx, 12% CO₂,6% oxygen, 2% water, and balanced by nitrogen. The flow rate was 944sccm, sorption pressure 12 psia, and sorption temperature 140.5° C.

Table 4 summarizes the in-flight test results of metal sulfide/substratesorbents from Table 1. All of the sorb ent samples were sieved through a325 mesh sieve prior to the injection. In-flight test results werecompared to samples made in accordance with the ion exchange methodsdisclosed in U.S. Pat. No. 6,719,828, which demonstrated a total Hgremoval between 70 to 90% (injection rate 6-10 lb/MMacf).

The data in Table 4 shows that the CuS/bentonite samples prepared byincipient wetness or solid-state grinding has the same or betterHg-removal than sorbents made by ion exchange methods.

TABLE 4 Selected In-Flight Test Results Rate Injection Hg d Sample Rateremoval (Hg %)/ # Sorbent* (lb/MMacf) (%) dt ECS-01 CuS/bentonite 8.4 930.133 10% Cu basis ECS-02 CuS/bentonite 8.4 93 0.133 10% Cu basis CuSO₄precursor ECS-03 Cu/bentonite 9.9 89 0.264 20% Cu basis ECS-04Fe₂S₃/bentonite 7.4 17 0.044 10% S basis ECS-06 (CuS + CuCl₂)/bentonite8.2 62 0.067 20% Cu basis; Cu:S = 2:1 ECS-07 (CuS + S)/bentonite 9.0 900.406 10% Cu basis; Cu:S = 1:2 ECS-08 MnS/bentonite 5.9 73 0.030 5% Sbasis ECS-09 Sulfur/bentonite 5.5 32 2.5 10% S basis ECS-10 Bentonite,As-is 5.0 27 0.021 ECS-11 MnS₂/bentonite 8.6 47 0.086 10% S basis ECS-12KMnO₄/bentonite 9.3 9 2.8 6% Mn basis ECS-15 CuS/bentonite 11.6 90 68.320% Cu basis dried at 150° C. ECS-21 CuS/bentonite 8.2 92 29.1 10% CuBasis ECS-22 CuS/bentonite 6.2 93 43.0 4% Cu basis ECS-23 CuS/bentonite7.2 76 24.6 4% Cu basis, CuSO₄ ECS-24 CuS/bentonite 8.8 89 22.3 7% Cubasis ECS-25 CuS/bentonite 7.3 62 22.0 7% Cu basis, CuSO₄ ECS-26CuS/bentonite 5.7 94 30.8 10% Cu basis (blender scale-up) ECS-27CuS/bentonite 7.7 94 47.4 10% Cu + 10 g MgCl₂ ECS-28 CuS/bentonite 10.394 20.5 10% Cu + 2.0 g MgCl₂ ECS-29 CuS/bentonite 7.8 36 6.2 10% Cu +5.0 g MgCl₂ ECS-30 CuS/bentonite 7.5 93 47.4 10% Cu, CuSO₄/CuCl₂ = 1.5ECS-31 CuS/bentonite 7.0 91 39.5 10% Cu, CuSO₄/CuCl₂ = 2.2 ECS-32CuS/bentonite 9.9 31 28.5 10% Cu, Cu acetate ECS-33 CuS/bentonite 7.8 5924.6 7% Cu, Cu acetate ECS-34 CuS/bentonite 11.5 23 18.9 4% Cu, Cuacetate ECS-35 CuS/bentonite 4.5 58 24.0 10% Cu, Cu nitrate ECS-36CuS/bentonite 6.3 54 14.6 7% Cu, Cu nitrate ECS-37 CuS/bentonite 5.3 5519.6 4% Cu, Cu nitrate ECS-38 CuS/bentonite 3.2 19 10.0 10% Cu, CuSECS-39 CuS/bentonite 3.6 32 14.1 2% Cu, CuS ECS-40 CuS/bentonite 6.1 9331.9 1% Cu, incipient wet ECS-41 CuS/bentonite 6.0 93 31.0 2% Cu,incipient wet ECS-42 CuS/bentonite 7.0 98 28.0 3% Cu, incipient wetECS-43 CuS/bentonite 5.9 36 12.3 1% Cu, CuSO₄, incipient wet ECS-44CuS/bentonite 6.9 31 12.0 2% Cu, CuSO₄, incipient wet ECS-45CuS/bentonite 75. 40 118.4 3% Cu, CuSO₄, incipient wet ECS-46CuS/bentonite 7.3 87 27.9 4% Cu, CuSO₄ + 0.1 gMgCl₂ ECS-47 CuS/bentonite7.7 83 31.1 3% Cu, CuSO₄ + 0.1 gMgCl₂ ECS-48 CuS/bentonite 7.5 80 33.53% Cu, CuSO₄ + 0.2 gMgCl₂ ECS-49 CuS/bentonite 6.2 81 27.1 3% Cu,CuSO₄ + 0.5 gMgCl₂ ECS-50 CuS/bentonite 6.4 87 30.5 3% Cu, CuSO₄ + 0.1gNaCl ECS-51 CuS/bentonite 6.7 83 27.4 3% Cu, CuSO₄ + 0.2 gNaCl ECS-52CuS/bentonite 5.6 89 18.1 (*unless stated otherwise, CuCl₂•2H₂O andNa₂S•9H₂O precursors were used)

FIGS. 1 and 2 further demonstrate the optimized copper loading andimportance of the presence of chloride anion in the sorbent. As shown inFIG. 1, at the Cu loading of 4 wt %, the Hg-removal seems to havereached the maximum level. Samples were prepared by the incipientwetness and solid state grinding techniques described using thebentonite substrate and the precursors noted in Table 1. The copperloading level was varied between 0 and 10% and the Hg removal wasmeasured as described above. Different metal precursors exhibiteddifferent Hg-removal efficiency. For the results shown in FIG. 1, onlythe sorbents made using a CuCl₂ precursor provided Hg-removal of above80%. The highest Hg removal was 98% at 3% Cu loading level, which is notshown in the Figures.

Adding a small amount of chloride, such as NaCl or MgCl₂, to coppersulfate significantly increases its Hg removal, shown in FIG. 2 whereall sorbents have 3% Cu loading level on bentonite substrate loaded withcopper sulfate precursors as described above and prepared by incipientwetness. FIG. 2 demonstrates that adding a small amount of chloride inthe sorbents effectively enhances Hg-removal. This also has practicalimportance. For example, the use of pure chloride salt precursor canlead to corrosion of stainless steel reaction vessels. Therefore, theaddition of mixed salts not only enhances mercury removal but reducescorrosion of equipment.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit or scope of the invention. Forexample, while the sorbents disclosed herein are particularly useful forremoval of mercury from the flue gas of coal-fired boilers, the sorbentscan be used to remove heavy metals such as mercury from other gasstreams, including the flue gas of municipal waste combustors, medicalwaste incinerators, and other Hg-emission sources. Thus, it is intendedthat the present invention cover modifications and variations of thisinvention provided they come within the scope of the appended claims andtheir equivalents.

1. A method of removing pollutants from a flue gas stream comprisinginjecting a sorbent into a coal-fired boiler flue gas stream, thesorbent comprising bentonite particles having a metal sulfide dispersedon the surface of the particles by a grinding, milling or incipientwetness process.
 2. The method of claim 1, wherein the pollutantsinclude mercury.
 3. The method of claim 1, wherein the metal sulfide isthe reaction product of a metal salt and a sulfide salt.
 4. The methodof claim 3, wherein the reaction product is an in situ reaction product.5. The method of claim 1, wherein the bentonite particles have anaverage size less than about 80 μm.
 6. The method of claim 3, whereinthe metal salts are salts selected from the group consisting ofalkaline, alkaline earth metals and metals having an atomic number inthe range of 21 to 30, 38 to 50 and 56 to 79, and combinations thereof.7. The method of claim 6, wherein the metal salt includes a metalselected from the group consisting of copper, titanium, tin, iron,manganese and mixtures thereof.
 8. The method of claim 6, wherein themetal salt is selected from the group consisting of nitrate, chloride,sulfate, acetate salts and mixtures thereof.
 9. The method of claim 1,wherein the loading level of the metal sulfide is in the range of about1 to about 20 weight percent.
 10. The method of claim 3, wherein thesulfide salt is a sulfide precursor that forms a S²⁻ anion.
 11. Themethod of claim 10, wherein the sulfide salt is selected from the groupconsisting of Na₂S and (NH₄)₂S.
 12. The method of claim 1, wherein themetal sulfide is selected from the group consisting of copper sulfides,tin sulfides, manganese sulfides, titanium sulfides and iron sulfides.13. A method of removing pollutants from a flue gas stream comprising:preparing sorbent particles by mixing a solid metal salt with bentoniteparticles, adding a sulfide salt into the mixture using a grindingprocess, milling process or an incipient wetness process so that themetal salt and sulfide salt react to form a metal sulfide on the surfaceof the bentonite particles, and drying the mixture; and injecting thesorbent particles into a coal-fired boiler flue gas stream.
 14. Themethod of claim 13, wherein the method does not utilize ion exchange toform the metal sulfide on the surface of the particles.
 15. The methodof claim 13, wherein the metal sulfide forms in situ on the particles.16. The method of claim 13, wherein the metal salts are salts selectedfrom the group consisting of alkaline, alkaline earth metals and metalshaving an atomic number in the range of 21 to 30, 38 to 50 and 56 to 79,and combinations thereof.
 17. The method of claim 13, wherein thesulfide salt is a sulfide precursor that forms a S²⁻ anion.
 18. Themethod of claim 13, further comprising reducing the average particle ofthe particles to less than about 80 μm.